When oil spills out in a harbor, the spilled oil has heretofore been prevented from spreading by the operation of encircling an oil-spilled sea surface with oil fences which is performed in the oil-spilled sea area by the crew members of an operating ship for towing and setting up the oil fences.
However, when oil accidentally spills out of a vessel in the open sea or in the ocean where tidal currents are more violent and waves are rougher than in a harbor, not only the operation performed by the operating ship is dangerous but also the operating ship itself may be capsized by accident especially in stormy weather. Under such circumstances, the operation of setting up oil fences and encircling oil spills with the oil fences is extremely difficult, and thus there exists a problem that effective measures to prevent the spreading of spilled oil cannot be taken.